In the field of displays there is a requirement for devices having a low power consumption. Devices made from liquid crystal materials have been shown to satisfy this requirement because they have a very large electrical resistance, and at the present time a considerable amount of interest is being shown in such devices for this reason.
There are many known liquid crystal materials; some have been known for many years. Liquid crystal materials are organic materials which exhibit a liquid crystal phase in which the molecules are arranged over limited spatial ranges in an ordered structure.
There are three known kinds of liquid crystal phase. One is known as a smectic mesophase in which the ordering of the molecules is of a lamellar kind. Another is known as a nematic mesophase in which a statistical ordering of the molecules exists parallel to the long axes of the molecules. The third is known as a cholesteric mesophase, in which the ordering of the molecules is of a helical kind.
Liquid crystal materials consist of compounds having a molecular structure which is elongated. For the liquid crystal compounds which have hitherto been used in devices the structure can be generalised and represented as follows: ##SPC3##
Where Q and Q.sup.1 are terminal groups and A and B are linkage groups.
Except in the case of certain esters where the unit A-B includes the group CO.O the unit A-B generally includes a double or triple bond. For example the Schiff's bases which are widely used in commercial liquid crystal devices, such as para-methoxy-benzylidene-butylaniline (MBBA) or para-ethoxybenzylidene-butylaniline (EBBA), are compounds containing the unit C = N.
The presence of a double or triple bond in the unit A-B leads to a chemical and/or photochemical instability of the material and is therefore undesirable for this reason. For example, Schiff's bases are readily hydrolysed, even by traces of water, yielding potentially toxic amines.
It had been supposed until recently that the double or triple bond in the unit A-B is desirable to confer rigidity on the molecule and to provide a reasonably high liquid crystal to isotropic liquid transition temperature together with a reasonably low crystalline solid to liquid crystal transition temperature.
Recently, it has been shown that reasonably low crystalline solid to liquid crystal and reasonably high liquid crystal to isotropic liquid transition temperatures can be obtained whilst maintaining low chemical and/or photochemical instability by using as a liquid crystal material one of the materials defined in U.S. copending application Ser. No. 413247. This application describes material consisting of or containing at least one compound having the formula ##SPC4##
where X and Y are various selected para-substituents.
It is an object of the present invention to increase the temperature ranges over which certain individual compounds having the said formula are liquid crystal.